In a high frequency power amplifier, it is necessary to stabilize characteristics of a transistor in order to suppress parasitic oscillation. As such a stabilizing method, there is a method of adding, to an input side of the transistor, a CR low frequency stabilizing circuit that is a circuit obtained by connecting capacitance C and resistance R in parallel.
In the CR low frequency stabilizing circuit, a high frequency component (a signal in a GHz range) passes through the capacitance C, and a low frequency component (a signal in a MHz range) that is a parasitic frequency passes through the resistor R. Consequently, only the low frequency component is lost, and thus parasitic oscillation caused by the low frequency component is suppressed.
For example, a high frequency/high output amplifier disclosed in Patent Literature 1 includes an input matching circuit for converting input impedance of a power transistor into high impedance, and a CR low frequency stabilizing circuit is provided in a preceding stage of this input matching circuit. With this structure, characteristics of the power transistor can be stabilized in a low frequency band.